We will continue to synthesize new compounds and evaluate these and others recently prepared, as aromatase (estrogen synthetase) inhibitors and for effectiveness in causing regression of carcinogen (DMBA)-induced, estrogen-dependent mammary tumors in the rat. Steroid and protein hormone measurements will be completed in blood samples from rats treated with each aromatase inhibtor. We will determine whether the inhibitors are enzyme specific and using tritium labelled compound, whether the inhibitors are aromatized themselves. Their metabolism in the tumor-bearing rats will also be investigated. Studies will be carried out to determine if the observed regression of estrogen-dependent mammary tumors with the compounds currently under investigation can be counteracted by a. estradiol b. prolactin. Estradiol receptor assays will be completed on tumors biopsied before and during treatment with inhibitors. BIBLIOGRAPHIC REFERENCES: The Effect of an Aromatase Inhibitor, 4-hydroxy-4-androstene-3,17-dione, on Estrogen Dependent Processes in Reproduction and Breast Cancer. Angela M.H. Brodie, William C. Schwarzel, Abubakar A. Shaikh and Harry J. Brodie. Endocrinology (1977), in press. Further Studies with Aromatase (Estrogen Synthetase) Inhibitors for Fertility and Mammary Tumor Control. Angela M.H. Brodie, D.A. Marsh, J.T. Wu and Harry J. Brodie. Endocrinology (1977) Society Abstract.